Practical joke is quietly disgusting (5)
I believe the answer is:
prank
'practical joke' is the definition.
(thesaurus)
'quietly disgusting' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'disgusting' becomes 'rank'.
'p'+'rank'='PRANK'
'is' acts as a link.
(Other definitions for prank that I've seen before include "Mischievous trick" , "High-spirited mischief" , "Mischievous act done for fun and amusement" , "Jape" , "April fool" .)